Solar Bike Project

Our first search was to find an efficient electric drive motor and a way for it to drive the bike. Since we wanted to be able to go up hills at optimum efficiency, it was attractive to use a chain-driving motor or “mid drive” that allows for gear changing via the derailleur. The best of these is made by Ecospeed of Portland, Oregon, which we saw operating firsthand on one of Brent Bolton’s bikes. We took careful measurements of the motor’s size and from these Russell made a foamcore model, which we used to figure out where in the hell the thing would fit on our existing bike’s frame. An obvious spot was in the triangle behind the captain seat and in front of the rear crank. But would it fit? The foamcore model proved it would, so we mounted the EcoSpeed motor to a quarter-inch thick aluminum plate connected to the frame with four stout U-bolts. Ah, it worked.

Key to using both human power and an electric motor is to make the motor independent of the crank, so the riders wouldn’t have to spin their legs. Each power source needs a freewheel, so we found an oddball freewheel gadget once made for Vision tandem bikes. It was called the IPS or Independent Pedaling System, basically freewheels that made the chainrings for the captain and stoker independent of each other. Thus we could drive a chainring on the rear crank from the motor, and another chainring on the same crank with the pedals, and make them independent (the motor having its own freewheel). We had some problems keeping the chainline aligned at first with the new freewheel, but eventually using a chain tensioner and structural reinforcement we got it right.

Every solar vehicle needs batteries, and to choose the right type we carefully calculated the amp draw and distance requirements we’d need to make for an efficient system. We’d need batteries to store surplus solar power to keep us going during cloudy weather, in tunnels, or when shaded by trees. More to the point, we knew that hill-climbing would demand more energy per hour than our solar panels could produce, so we needed as large of a buffer as we could carry. That battery would have to be charged and discharged quickly, maybe 50 amps of current drain over some tens of minutes, so that narrowed the available (and affordable) battery chemistries.

In the end we settled on a new design of the old standard lead-acid battery. We began testing our system using two Optima yellow-top deep-discharge batteries, 55 amp-hour capacity. The batteries are heavy, at 42 pounds each, and they store only about 15 watt-hours per pound. But they’re just what we need – they will quickly accept a full charge in just two hours of morning sunlight and then charge up again in a couple of hours at the end of the day. In the meantime, we set about designing our system so that it would run almost-completely on solar power in the middle of each day.

We needed hard numbers to get our electrical and mechanical design right, so we pressed into service a 20-year-old Burley child trailer and used it to carry the test batteries. We mounted a dedicated charger to each battery box so we could easily lug each battery to a separate 20-amp circuit when we ran out of juice during our first field runs using only battery power and no solar panels. Using this arrangement, in 2007 we performed numerous “data runs” up and down the tree-lined streets of midtown Sacramento. We would test the power draw for various gear ratios and speeds, finding the optimum combinations. Neighbors got used to seeing these two guys calling out to each other strings of number of amperes, volts and speed as we created a spreadsheet of data on the run. We used our knowledge to adjust the size of the sprockets on the motor and crank, so that efficiency was maximized at the speeds we wanted, while we could pedal at any speed to add power. With all this data we could estimate the performance with a given solar panel’s power. We learned that we needed something between 500 and 800 watts to propel our beast continuously at 20 miles per hour. Could that much solar power fit on our bike? And how could we possibly afford so many solar panels, we wondered.

With the tiny amount of overhead square footage on a bike and trailer, we really needed to find the very most efficient solar cells available. And because our project is dedicated to using off-the-shelf parts rather than aerospace techniques like custom-wiring hundreds of individual cells, we looked for the most efficient solar panel maker. That led us to San Jose, California-based SunPower Corporation.

Bryan located a key executive, Bobby Ram at SunPower and explained the design challenge. At the time, young SunPower didn’t yet have a community donation program or any standard way of dealing with our request. They were moving offices, expanding greatly and becoming the US leader in PV power. But they had the best panels in the business.

We went to their headquarters, presented our design (fully realized in Photoshop), and retuned some months later to pick up three SPR-315, 300W panels each 19% efficient. These are standard panels normally used for rooftop residential PV systems. (Thanks, Bobby!) Compare the 19 percent SunPower efficiency with 12-16% for most other single-junction silicon panels. The Sunpower cell design is unique, with the positive and negative electrodes interdigitated on the rear surface and the cell made from high purity material. Photocarriers from sunlight aren’t reabsorbed as quickly in this material, and have a greater chance of reaching the electrodes (which are in shallowly doped regions to realize PN junctions), resulting in greater efficiency. With an astonishing 945W of solar, we now could count on wattage comparable to the million-dollar race cars and to the best of the other non-race projects like Solar Taxi and Xof1. As it happens, the 315 watts each panel is capable of producing is roughly equal to the average energy output of a champion Tour de France cyclist. Imagine our good fortune to have three SunPower panels, or the equivalent of having three such powerful athletes silently helping us pedal our bicycle.

The SunPower panels are beautiful and rugged, able to withstand 50 mph hailstorms with hailstones an inch in diameter. But that makes them heavy, at 50 pounds each. Our design challenge was to find a way to mount the panels absolutely as low as possible to minimize any over-tipping of either our bike or trailer. So we decided to put one panel on the trike and two on a yet-to-be-designed trailer in back with the batteries. We designed a special low-center-of-gravity trailer together with Rich Porras of Sacramento’s Grease Kings, a biodiesel conversion shop. Rich helped us puzzle through some of the arcane design possibilities and in August 2008 welded the trailer out of surplus round and square aluminum tubing, and we supported the frame on two BMX shock-absorbing forks for suspension. Though it’s 10 feet long, the trailer frame is so light that can be easily picked up with one hand. It connects to the trike via a spherical bearing that allows motion in any rotational axis. Its 20 inch wheels are the same as those on the trike, so we carry only one kind of spare tire and tubes.

One interesting electrical challenge is that the SunPower panels need to be isolated from the negative side of the circuit, so we mounted them on insulating PVC tubing and pieces of Sorbothane. We were able to make the panels tiltable, which vastly increases the amount of energy we can capture in the early morning and late afternoon. Tilting up the panels also gives us access to the 15 cubic feet of trailer storage space, essential for getting at our tools, camping supplies and the charge controller. As the 42-inch-wide panels are wider than normal doors, this tilt-up feature also allows the bike and trailer to ease through a 36 inch wide door.

We’ve learned that to get maximum efficiency in using solar panels it’s best to use a maximum power point tracker, a device which gauges battery charge and adjusts the solar charging voltage to the most appropriate value. We wired up an Outback MX-60 charge controller and adjusted its bulk and float voltages so that the charger feeds maximum current to the motor even when the batteries are fully charged. The Outback displays input (solar) and output (battery/motor) voltage, current and power, which is imaged by a CCD camera and sent to a video monitor above the captain. Knowing what’s going on with battery charge, watt-hours used and a host of other measurements is critical as we develop and operate the bike, so we secured a DrainBrain (now sold as CycleAnalyst) by the Renaissance Bicycle Company of Vancouver BC which is useful in optimizing throttle control as well as monitoring performance. In operation, one adjusts the throttle during acceleration and cruising to keep the power below a desired level.

One key goal of our design was to have a sleek silent vehicle so we could enjoy day-long solar-powered tours of the countryside. We noticed immediately that the gear whine of the first planetary reducing gear of our chain-drive motor emitted lots of noise. By late 2008, though, Brent Bolton of EcoSpeed was able to procure a much-quieter and more-rugged planetary gear unit which made the sound much quieter. But in the meantime we’d fallen in love with a silent and powerful hub motor made by Crystalyte. We knew we could only use the 5303 ungeared motor efficiently at our top cruise speed of 18-20 mph, but the silence and simplicity of it led us to add it to our system and mount it in the rear fork of the Greenspeed trike. We bought the Crystalyte motor without a controller, figuring, a little too-cleverly, that we could use the EcoSpeed motor’s controller to power it. Immediately we found the hub motor drawing 1.3 kilowatts, far in excess of what it should, and heating up the poor motor controller so much it shut down smoking hot after only a few minutes of travel. Eventually we purchased a dedicated motor controller designed for the Crystalite, the 4840. Unfortunately the controller was designed to be operated at 48 volts and our system bus voltage is 36 volts. Happily some hurried email conversations with dedicated hobbyists on the “V is for Voltage” forum suggested a way we might modify the controller to work at 36 volts. With improvised tiny soldering iron Russ pulled out a resistor and mounted a variable potentiometer outside the unit we could adjust for varying lower cut-off voltages. Now the controller works beautifully at 36 volts.

With the right controller in place, the hub motor is efficient and ultra quiet. Now we have two usable motors and can switch between them by switching the throttle from one to the other. An excellent use for the two motors is to come up to speed with the geared-down mid-drive and then cruise with the hub motor in its efficient speed. What could be more extravagant? Our tests of the bike in 2009 shows that all our gearing and motor experiments paid off with high efficiency and luxurious quiet. Depending on our speed, the touring bike, weighing in at 860 pounds including riders, consumes only between 30 and 50 watt-hours per mile, just exactly what’s available to cruise all day on the output of our three solar panels.

When we added the hub motor capability in late 2008 and incorporated our new long solar trailer, we changed from a bus voltage of 24V to 36V for additional power. This caused a little problem -- how to run many of the extreme 24-volt visibility lights on 36 volts. The solution: wire in a salvaged DC-to-DC converter to generate 24V from the 36-volt bus. These 24-volt lights were no mediocre bike lights, by the way. To make us visible we designed and built a super-efficient headlight out of six, 3W Luxeon LEDs, with reflectors and diffusers. This 20-watt headlight is at least as powerful as two standard 55-watt incandescent auto headlights, allowing us to ride at night on reserve battery power. Because we’re determined to be seen by all road vehicles, we’ve also attached a yellow xenon strobe flasher (originally designed for Caterpillar machines on the bike’s topmost point. Add to that some yellow LED running lights on the sides and brilliant red LED light bars flashing on the rear surfaces saying “bike ahead” with the intensity of a car taillight. We have yet to attach 24-volt white strobes from surplus room fire alarms to the sides, to become a rolling construction site. Maybe planes will land on us.

Seriously, though, we’ve designed the solar touring bike to fit the usual legal status of an electric-assist bicycle. Most state vehicle codes will allow such electric-assist bikes to travel up to 20 mph and use no more than 750 watts of power. We’ve been scrupulous to adhere to these guidelines, as well as incorporating the lighting, bells, horns and visibility flag to comply with local ordinances. As an electric-assist bike our solar tourer can travel anywhere bicycles are permitted, and does not require an operator’s license.

THE ADVENTURE AHEAD

After a year of weekend garage construction and dozens of shakedown rides we’re now poised to take the solar touring bike on the long adventures we built it for. Prime solar touring season lies between April and September each year, so we’re working toward a multi-state tour beginning in northern California sometime during that time frame. Depending on our day-job work schedules we don’t yet know if we will be able to make the 1,500-mile solar tour in 2009 or 2010. In the meantime this spring we’ll be putting final touches on the bike’s mechanical and electrical systems. Look for us at one of our first public exhibits -- the 2009 Maker Fair Bay Area, May 30 and 31, at the San Mateo County Expo Center.